A father and his two sons are facing life in prison after they were found guilty yesterday of the honour killing of a university student who was stabbed 46 times.

Chomer Ali, 44, ordered his two sons, Mohammed Mujibar Rahman, 19, and Mamnoor Rahman, 16, to kill his daughter's boyfriend, Arash Ghorbani-Zarin, 19, last November after he discovered that she was pregnant. Manna Begum, 20, had been dating the victim against the wishes of her father, who had set up an arranged marriage and he was killed "to vindicate the family's honour".

Mr Ghorbani-Zarin, an Iranian Muslim studying electronic engineering at Oxford Brookes University, was attacked as he sat in his car. The jury at Oxford crown court was told that Mamnoor's DNA was found on the butterfly knife used to kill him and that bloodstained clothing belonging to the brothers was found in a plastic bag.

Ms Begum - described as intelligent, independent and strong-minded - had been promised by her father, a traditional Muslim, to another man, but she continued to date Mr Ghorbani-Zarin. The couple were described by friends as devoted to each other. They wanted to marry and did nothing to hide their affection, often holding hands. This, the jury was told, "brought shame on the family".

Summing up the case, Mr Justice Gross said the relationship had caused a battle of wills within Ms Begum's family, where there was already a cultural divide between the Bangladeshi-born father and his British-born children. The relationship "broke religious and family taboos".

Mujibar was furious at his sister's "blatant" defiance and slapped her three times when she refused to end the relationship. In a police interview, he explained: "She acted contrary to religion and tradition by dating Arash. Instead of dating, she should have waited to have an arranged marriage."

Julian Baughan QC, prosecuting, said: "Their relationship brought shame and dishonour on the family. That drove the accused, headed by the head of the family, to murder Arash to vindicate the family's honour. It is inconceivable that a murder of this kind would have been organised by his 15 and 18-year-old sons [their ages at the time] on their own. It had all the hallmarks of a family killing with the father egging the boys on."

Mr Justice Gross adjourned sentencing for reports on the youngest defendant, Mamnoor. The three will be sentenced to life imprisonment for murder but a minimum term has yet to be decided.

There were shouts of delight when the verdict was announced and later the family of Mr Ghorbani-Zarin issued a statement through police. They said: "Our son was intelligent and loving and had a wonderful lifetime ahead of him. We miss him every second, every minute and every hour of every day. His death has crushed our family. This verdict gives us back some comfort that justice has been done but it does not bring him back."

Detective Chief Inspector Steve Tolmie said the murder was "a horrific, violent attack on an innocent young man who had a lot to live for. No one has the right to take away someone's life, for whatever reason, and people who choose to do this will be punished, as shown today."

Witnesses told the court how a man in a red Toyota, said by the prosecution to be Ali, threw a bag over a hedge into an Oxford allotment three days after the killing. In police interviews Mujibar admitted that the butterfly knife used to kill Mr Ghorbani-Zarin belonged to him but he claimed that it had been taken from his bedside drawer before the killing.

Ms Begum was introduced to Mr Ghorbani-Zarin by school friends in Oxford, where they both lived, in the summer of 2003 and became pregnant last August. She had an abortion after the murder.

Mr Ghorbani-Zarin told friends in late 2003 that Ali had threatened him with a knife. In November 2003 Ms Begum, who had been removed from school by her father and had her mobile telephone confiscated, slit her right wrist. When paramedics arrived at the door, the family refused to answer, instead pushing their daughter outside, causing her to fall over.

Ms Begum ran away from home to stay with the aunt of a friend. Friends said Mr Ghorbani-Zarin showed them a scan of the unborn child and took a job to support his girlfriend and the baby. Just weeks later he was found stabbed to death just 500 metres from his girlfriend's home.

Jomsed Ali, 60, a member of Oxford's Bangladeshi community, said the actions of Chomir Ali and his sons had been condemned. "What he did is very bad, for his family and his community. This is the first time I have ever seen this. I have never known it in Bangladesh either."